1. Field of the Invention
The present invention relates to a thermal head, and more specifically, it relates to a thermal head which has a plurality of thermal elements and drive circuit elements for controlling the thermal elements through electrical conduction in accordance with a print signal, where each of the drive circuit elements drives the thermal elements corresponding to two print dots on the basis of time-division.
2. Description of the Prior Art
Conventionally, with a line-type thermal head having a plurality of thermal resistors, many of them are of the type in which a single drive circuit element D drives a single thermal resistor R, as shown in FIG. 6, or a single circuit element B drives a plurality of thermal resistors R, using a blocking diode D, as shown in FIG. 7.
With the thermal head of the latter type, however, it does not require as many drive circuit elements D as the thermal head of the former type has, but requires the block diodes B as many as the thermal resistors R and a switching circuit for common electrodes VH1, VH2, etc.
Allowing for the problems mentioned above, a 1/2-dynamic drive system in which no blocking diode is used, as shown in FIG. 8, is invented. In this system, a single drive circuit element D drives two thermal resistors R on the basis of time-division, as shown in FIG. 10.
FIGS. 10(a) and 10(b) are diagrams showing currents I1 to I4 flow in thermal resistors R1 to R4 when drive circuit elements Tr1 and Tr2 turn ON and OFF, respectively. FIG. 10 (a) is a circuit diagram showing a case in which drive voltage VH is applied to a common electrode VH1 of the odd thermal resistors, while no voltage is applied to an even common voltage VH2: since an element Tr1 turns ON, I1&gt;&gt;I2 is satisfied, whereby the thermal resistor R1 heats up to be ready for printing, while the thermal resistor R2 does not heat up. The currents I3 and I4 flowing in the thermal resistors R3 and R4 satisfy the relations, I3=I4.apprxeq.I1.times.(1/2) because the element Tr2 turns OFF. Thus, the thermal resistors R3 and R4 heat up by approximately a quarter of the heating amount of the thermal resistor R1, and this makes no contribution to printing. FIG. 10 (b) shows a case in which drive voltage is applied only to the common electrode VH2, where since I1=I2.apprxeq.I4.times.(1/2)&gt;I3 is satisfied, the thermal resistor R4 alone is ready for printing.
As has been described, using a phenomenon that current flows in the thermal resistors not involved in printing by a half of the current flowing when they are driven but thermosensible paper is not color-developed by the current, no blocking diode is necessitated. In driving them, first drive voltage is applied to the common electrode VH1, and the drive circuit elements are turned ON/OFF corresponding to odd print dots to drive the odd thermal resistors. Then, drive voltage is applied to the common electrode VH2, and the drive circuit elements are turned ON/OFF corresponding to the even print dots to drive the even thermal resistors. In this way, a single line printing is carried out.
FIG. 9 is a diagram showing a main part of a wiring pattern of the thermal head shown in FIG. 8. The odd thermal resistors R2n-1 are connected to the common electrode VH1, while the even thermal resistors R2n are connected to the common electrode VH2, but since they cannot be wired in a single layer pattern, a layer insulating film F is formed between the common electrodes VH1 and VH2.
In the above-mentioned prior art embodiments, the embodiment shown in FIG. 8 is composed of the smallest number of components, but it is not so advantageous in price because a layer insulating film must be formed. Instead of forming the layer insulating film, there is proposed an idea that thermal resistors are formed on heat-resisting resin substrate and wired with through-holes. However, with the circuit shown in FIG. 8, the through-holes must be formed every other thermal resistor, and thus it is very difficult to form the through-holes, allowing for the pitch of the thermal resistors (e.g., 125 .mu.m). Additionally, if possible, the number of the through-holes is excessively large to lose any merit in price.